The present invention relates generally to MR imaging and, more particularly, to a system and method for tracking and visualizing interventional devices, such as catheters, via MR imaging to orient and guide real-time procedures without the need for localizing or embedded RF coils.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. Such techniques have increased the rapidity of image reconstruction to the point where real-time or near real-time imaging can take place.
Interventional and invasive medical procedures, in which implements are manipulated within a scan subject, are instances in which real-time MR imaging is particularly useful. It is often helpful to provide tracking and visualization of the position and orientation of interventional implements and devices to quickly and efficiently conduct these procedures.
Initial methods for visualizing and guiding interventional devices during medical procedures relied on a medical professional's ability to guess the orientation and location of an interventional device from a number of 2D images and background knowledge of subject anatomy, for both active and passive devices. In these methods, MR images of the implement are taken from a number of angles and displayed to the user. From these images, a user had to mentally interpolate the 3D orientation and location of the device with respect to the surrounding and target anatomy. Such methods are inconvenient for doctors and have slow response times to device movement because multiple images at various angles must all be continually refreshed.
Currently the most commonly used active method for interventional device visualization and guidance involves the use of embedded RF coils. The implements used in an interventional procedure are specifically created to contain a number of tiny RF coils (on the order of a few millimeters) embedded therein to provide a way to localize the position of the implements. MR signals acquired along multiple axes are used to localize the RF coils embedded in a device. At best, this method may be modified to visualize an approximated extended length of the device by embedding a large number of RF coils along its length. Nonetheless, MR images of devices incorporating RF coils appear as a series of bright dots from which the body of the device is generally extrapolated or assumed. The drawbacks of this method are that embedding micro-coils in medical implements constrains their geometry and mechanical properties when precision is important. While increasing the number of coils used improves the signal received, reducing the number of coils to allow flexibility in device geometry may result in insufficient localizing information. Thus, a tradeoff exists between geometric/mechanical device precision and imaging/localizing precision. Such imprecision in either form is undesirable for procedures that require tracking of multiple points and precise prediction and display of device response to user interaction.
However, in addition to device localization, visualizing extended lengths of devices is useful in interventional procedures. Visualization methods that make extended lengths of interventional devices appear bright with respect to the background have been performed using both passive and active techniques. Passive visualization techniques, such as filling a catheter with contrast agents like gadolinium, have inherently lower signal to noise ratios as compared to active imaging techniques. Such techniques also impose mechanical limitations on devices—if the lumen is filled with a contrast agent, it can't be used for additional device or substance delivery which may be essential to the procedure.
Extended lengths of interventional devices can be made to appear brighter with respect to the background by using other active techniques. For example, some methods utilize active RF antennas for improved visualization in MR images in the form of an elongated loop style or loopless style, and provide increased visual contrast in reconstructed MR images of extended length of devices. Various interventional devices such as catheters may be formed from, or may include, active RF antennas where extended lengths of devices are better visualized.
Active antennas that make extended lengths of devices appear bright are preferable since they are more conspicuous in MR images as they move within the body (as compared to passive techniques), yet do not increase device size (as do embedded RF coils, which are also active antennas). However, while these active antennas, which do not utilize embedded RF coils, can be used for imaging and imaging-based tracking methods, signals from these antennas are not generally usable to determine the location of a device within a body. For example, a user could not generally rely upon a localizing signal from the antenna to guide the MR acquisition scan plane thereto. Some methods exist wherein a combination of continuous active antennas and RF localizing coils are employed, but no method is presently known in which an entire or continuous length of an interventional device can be imaged and localized, without the use of embedded RF localizing coils. This same problem of localization also exists for passive devices, such as gadolinium filled catheters, where an extended portion of the device can be visualized but not localized.
Inherent to most current visualizing and tracking methods is the inconvenience of lost depth information. That is, depth information is generally lost in MR images of interventional devices (acquired by any of the above-mentioned techniques), causing a sometimes misleading device appearance. When an image of a device is overlaid on a “roadmap” or anatomical reference image, the entire device appears in front of all the anatomy in the roadmap image. Some adaptations have been developed to deal with these issues, but still present drawbacks. Computing 3D shapes from 2D projections, using fast bi-planar approaches and slower projection reconstruction approaches can be used to eliminate the depth anomaly that occurs when projection images are displayed on roadmaps. In such methods, device projection images are artificially blended semi-transparently into the reference image after acquisition so that the parts of the device existing below the reference image slice appear dim, while the portions above the reference image slice appear bright. However, such methods are not intended for, and do not provide, guidance of an interventional device for manipulation during a procedure. That is, the MR acquisition scan plane is not adjusted to track the interventional device or aid in interventional device placement.
It would therefore be desirable to have a system and method capable of directly and precisely obtaining the orientation and location of interventional devices with respect to the surrounding anatomy without the use of localizing RF coils. It would be further desirable for such a system and method to provide visualized guidance in real-time of device plane images with actual target anatomy so that operators will not be required to depend so heavily on experience with scan subject anatomy, device geometry, and device characteristics.